The resin transfer molding (RTM) process has become a popular composite manufacturing process due to its suitability for high volume production and cost effectiveness. In this process, a dry fiber reinforcement (preform) is enclosed in the mold and resin is injected and allowed to cure. Resin injection or transferability is defined by the permeability of the preform and it can be severely affected by defects, distortions or anomalies in the preform. In some cases, such as those involving complex geometries, it is not uncommon for a relatively flexible (non-rigid) preform to be misplaced or to shift and be distorted during mold closure. For example, a draping, shearing, and/or folding defect of such preforms has been observed to occur during placement of the preform in the molding cavity. Such a defect, if not detected and corrected before resin injection, results in costly scrapping of the finished part. Such preform defects can cause local permeability non-uniformities and affect the resin flow resulting in local resin-starved areas. Furthermore, preform distortions could contribute to residual stresses and undesirable stress concentrations during subsequent loading in service.
The impregnation of the fluid into the preform is defined by the permeability of the reinforcement, which is the ability of a Newtonian fluid to permeate a porous medium with a sufficiently low Reynolds number, as given by Darcy's Law. The permeability of an undistorted preform with a constant fiber volume ratio can be assumed to be uniform over the entire domain; however, the permeability can be significantly altered by defects, distortions, or other anomalies in the preform. Such drastic changes in local permeability can affect resin flow patterns, rendering portions of the mold to be insufficiently filled. Permeability variations within a preform can be attributed to a number of factors, such as improper preform preparation, misplacement or shifting in the mold, accidental inclusion of foreign material, natural surface density variation of the preform, etc. If such occurrences were not detected prior to resin injection, the potential for costly part scrapping would be increased. Aside from the additional voids due to permeability non-uniformities, preform distortions could contribute to residual stresses and stress concentrations during in-service loading. Early detection, therefore, of such reinforcement irregularities is critical for effective quality control.
To avoid difficulties associated with use of flexible (non-rigid) preforms, relatively rigid braided or stamped fibrous performs have often been employed for resin transfer molding. Such performs are more solid-like and easier to place in the molding cavity without distortion of the preform. However, it is still important to detect defects and irregularities associated with such relatively rigid braided or stamped fibrous preforms as a quality control step for reducing costly scrapping and insuring the quality and reliability of fabricated composite structures.